Restricting SMTP delivery per domain

[justneedadrink]

I have a plesk server which hosts several virtual domains. However there are some clients who would like to employ a 3rd party spam filtering service. What I would like to do is to be able to restrict the receipt of SMTP messages for a domain so that it can only receive emails from a certain host(s) while the rest of the domains can receive email from any other host. Does any one know how this may be accomplished?

Further information:

• 
  I want domain A to only recieve emails from IP addresses 1.2.3.4 and 2.3.4.5
  All other domains should be able to receive mail as usual from any IP address